Sonic detection method and apparatus

ABSTRACT

A sonic detection system installed adjacent the upstream side of a bridge for detecting and indicating the intrusion of swimmers into an adjacent guarded volume of water. The system utilizes the interference signal attributable to multiple signal reflection paths and Doppler frequency shifts therein to enable the detection of both swimmers floating with the primary current and those moving relative to the primary current. The system includes a directional, continuous wave sound projector and a matched directional receiver which are submerged and are arranged in spaced relation normally of the primary current to have their beam pattern axes intersect at a point upstream of the bridge for establishing the guarded volume. Fixed filters and an adjustable filter are included along with a display which is adjustable for the prevailing ambient conditions to provide indications that the overall response and/or certain received signal frequency spectra have increased above generally randomly varying ambient levels.

United States Patent [1 1 Suter June 19, 1973 1 SONIC DETECTION METHODAND APPARATUS Primary ExaminerRichard A. Farley AttorneyE. J. Brower, H.Hansen and B. F. Buchan, Jr.

[57] ABSTRACT A sonic detection system installed adjacent the upstreamside Of a bridge for detecting and indicating the intrusion of swimmersinto an adjacent guarded volume of water. The system utilizes theinterference signal attributable to multiple signal reflection paths andDoppler frequency shifts therein to enable the detection of bothswimmers floating with the primary current and those moving relative tothe primary current. The system includes a directional, continuous wavesound projector and a matched directional receiver which are submergedand are arranged in spaced relation normally of the primary current tohave their beam pattern axes intersect at a point upstream of the bridgefor establishing the guarded volume. Fixed filters and an adjustablefilter are included along with a display which is adjustable for theprevailing ambient conditions to provide indications that the Overallresponse and/or certain received signal frequency spectra have increasedabove generally randomly varying ambient levels.

' 19 Claims, 6 Drawing Figures W K H I 65 05C 1 {ah/i Vflfilagflilorg/mm v w /r 541 t DISPLAY CONT/70L 6O 63 OR 61 ADJUSTABLE 57 ETHRESHOLD 44 RESPONSIVE \fl I 559 I DRIVER I T l as ii 5 e2 91 7 5e 7662 n 45 M f IIAGC a ixhlaiA s s i kt D'SPLAY BF\ROJECTORI 2 FILTER 2 4CONTROL I as 92 8 B7 as ENVELOPE 72-, l 17 l O-TARGET DETECTOR 7 15-20Hz 3 DISPLAY BANDPASS 1 kt. CONTROL FILTER 4 41 e4 93 RECEIVER a 87 m 13I is 20-21 H1 1 DISPLAY BANDPASS 1 1- kt, 33 34 as FILTER "l a CONTROLBANDPASS e1 FILTER a 19 J 27-36 H! BANDPASS 1-;- 1% kt. EBi J FILTER as95 15- El so 8' 16 56-45 H: l 4 z DISPLAY BANDPASS 1- 2- kt.

FILTER 5 5 CONTROL DISPLAY l J aivfl 1 SONIC DETECTION METHOD ANDAPPARATUS STATEMENT OF GOVERNMENT INTEREST The invention describedherein may be manufactured and used by or for the Government of theUnited States of America for governmental purposes without the paymentof any royalties thereon or therefor.

BACKGROUND OF THE INVENTION In general, this invention pertains tounderwater detection systems and, more particularly, to a continuouswave, Doppler sonar system.

The threat of destruction of harbors, bridges, floating barracks and thelike by underwater attackers demands a high degree of constantsurveillance by. security personnel. The effective degree ofsurveillance is substantially reduced by the difficulty of positivelydetecting the approach of underwater attackers particularly at night orduring rainstorms. Conventionally used defenses such as nets, fences andthe like are vulnerable SUMMARY OF THE INVENTION It is the generalpurpose of this invention to provide a simple, portable, low cost sonicdetection system which is easily adapted to provide an automatic alarmand which can be used by unskilled personnel as an aid in maintaining aconstant, effective surveillance which protects installations accessibleby water from enemy attack. Briefly, the general purpose of theinvention may be accomplished by providing a continuous wave sonarsystem including a directional acoustic signal projector and adirectional receiver arranged in spaced relation adjacent the upstreamside of the installation with intersecting beampatterns establishing aguarded volume of water spaced from the projector and receiver andincluding a signal processor continuously monitoring the complex signalreceived while continually ensonifying the guarded volume, which signalprocessor has adjustable and fixed filters and display means forindicating increases above ambient levels of the received signal andselected frequency spectra thereof.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 4 and 5 represent wave analyzerrecordings of the relative amplitudes of various Doppler shift spectrafrequencies processed from a received acoustic interference signal; and

FIG. 6 represents a block and schematic diagram of the sonic detectionsystem according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 'In general, the system mode ofoperation contemplates that a continuous wave acoustic signal includingDoppler shifted signal spectra attributable to relatively movingreflective objects be processed as by filters to obtain indications ofamplitude increases above ambient levels of certain Doppler frequencyspectra. Such increases of certain selected Doppler spectra are used toindicate the intrusion of floating or swimming underwater attackers intoa guarded volume of water.

Referring now to FIG. 1, the system generally includes a transducerarray 10 having a directional, single tone, continuous wave acousticsignalprojector 11 and a directional acoustic signal receiver 12. Moreparticularly, the projector 11 and the receiver 12 are sub merged in astream 13 in spaced relation to each other and in fixed relation to thestream bottom on the upstream side of a water accessible installationsuch as a bridge 14. The projector 11 and receiver 12 generally includematched spherical transducers, such as 15 of FIG. 2, which are fixedlymounted at the foci of circular paraboloid reflectors, such as 17 ofFIG. 2, which are each conveniently made of an aluminum dish internallycoated with sound reflecting material such as Corprene comprisingbonded-together cork particles. The coating is configured to form aconcave parabolic acoustic signal reflecting interface. The specificshape of the paraboloid is determined by the desired directional beampattern such as A and B having respective axes a and b for the projector11 and receiver 12. One suitable narrow-beam pattern for the projectorand receiver beams A and B is one which is down 3 db at: 8.5 relative tothe beam axis, down 10 db at i 15, down 20 db at i 20 and down 40 db at25, all side lobes being reduced to a minimum.

For design purposes it is postulated that underwater attackers will mostprobably attempt to float or drift down with the stream current to asupporting structure of the bridge 14 such as a piling l9 and affixexplosives thereto for destroying the bridge 14. Therefore, theprojector 11 and the receiver 12 are symmetrically spaced apart adistance C on respective sides of the piling 19 along a line extendinggenerally normal to the anticipated direction of the primary current ofthe stream 13, the axes a and b of the narrow-beam pat terns A and Bgenerally intersecting at a position D ar ranged on the perpendicularbisector of the spacing distance C and upstream from the piling 19.Since the projector ll continually ensonifies the upstream area with thecontinuous wave carrier signal of a selected frequency, the specificallyarranged beam patterns A and B of the projector 11 and receiver 12 forma guarded volume G of generally elongated shape which extends across theanticipated paths which an underwater attacker might use in approachingthe bridge 14 to destroy the piling 10.

More particularly, the beam axis intersection D is arranged a sufficientdistance upstream such as 50 to 60 feet for a prevailing primary streamcurrent of one knot to ensure that an intruding underwater swimmer canbe detected in sufficient time for appropriate countermeasures to betaken. If the spacing C between the receiver 12 and the projector 11 istoo small and if the beam patterns axes a and b intersect at too largean angle, i.e., much greater than 20 to 30, it has been observed that aDoppler signal of substantial amplitude most probably attributable tothe reflections from the adjacent moving water and discontinuitiestherein affects an impermissably low signal-to-noise ratio.Additionally, if the projector 11 and receiver 12 are arranged too closeto each other, more energy is reflected which includes Doppler shiftedsignals caused by back scattering from the adjacent water surface andnearby discontinuities. By spacing the projector 11 from the receiver 12at distances such as at least feet or greater and by using narrow,directional beam patterns such as that described, the portions of thereceived signal attributable to reflections from the adjacent portionsof the water surfaces are attenuated a sufficient degree to reduce theamplitude of such ambient Doppler shifted spectra to acceptably lowlevels permitting detection of intruding targets.

For example, one suitable configuration includes a projector-receiverspacing C of 10 feet with the beam axes a and b being deflectedtherefrom at angles of 85 to intersect at the point D located about 57feet away from the piling 19. In this way, even though the guardedvolume G has a relatively narrow width, the Doppler shift in thereceived signal attributable to the moving water very closelyapproximates the DOppler shift which would occur if the projector 1 land receiver 12 were both located at the piling 19. More importantly,the range of the described array 10 extends upstream a great distancesuch as 800 feet for quiet water and 300 or 400 feet for a stream 13having a primary current of about one knot. Another suitableconfiguration includes a projector-receiver spacing C which is equal to50 feet, the beam axes, a and b, being deflected therefrom at 80 anglesto intersect at the point D located about 142 feet from the piling 19.Where a long span is to be protected, the arrays 10 of several detectionsystems can be utilized to preferably establish contiguous guardedvolumes, one array 10 being positioned adjacent at least each support.

Referring to FIG. 2, it is seen that the projector 11 and receiver 12are mounted on tripod stands such as which are fixed to the bottom ofthe stream 13. A spacing reference comprising a plurality of end-to-endlinked, rectangular aluminum panels, the end panels being scored with anappropriately arranged beam deflection reference line, can be used tofacilitate precise positioning and orientation of the projector 11 andthe receiver 12. It is generally desired that the projector 11 and thereceiver 12 be located near the bottom below the elevation of theanticipated paths of approaching underwater swimmers so that the beamscan look generally upwardly at a slight incline toward the moving targetto be detected. However, it has been discovered that the character ofthe bottom of the stream 13 effects the operation of the detectionsystem in that rocky bottoms tend to directly reflect more energy to thereceiver 12 than do sandy bottoms. Therefore, it may be necessary toposition the projector 11 and the receiver 12 further above rocky streambottoms in order to reduce the amplitude of unwanted ambient Dopplershift spectra.

In order to enhance understanding of the detection system, FIGS. 3, 4and 5 are provided to illustrate both the type of interference signalbeing received and certain examples of frequency spectra amplitudeincreases which provide a basis for detecting both floating swimmers andthose which are moving relative to the primary stream current.

FIG. 3 represents the envelope of a mechanical amplitude-time recordingof the complex acoustic interference signal received during severalsuccessive time segments in which an intruding underwater swimmer isapproaching the transducer array 10 through still water. The first timesegment t t, of the recording of FIG. 3 generally discloses the receivedsignal during ambient conditions before the underwater swimmer entersthe guarded volume. It is to be noted that the amplitude of the receivedsignal generally has a random variation occasionally interspersed withsharp spikes. The random variation in the signal amplitude is thought tobe attributable to the ever-changing geometry of the multitide of raypaths of acoustic signal transmission and reflection. Since, as shown bythe paths x, y and z in FIG. 2, the projected carrier signal isreflected from the water surface and from a multitude of fixed objectslocated at varying distances on or adjacent the bottom of the stream 13,the reflections from the several fixed objects may each differ in phase.As surface wavelets constantly change the ray path lengths, changingphase differences in the multipath reflections collectively causerandomly occurring cancelation or reinforcement of the projected carriersignal. During the time segment t t;,, a swimmer entered the guardedvolume G at a range of about 360 feet as is generally indicated by thegenerally increased average amplitude of the complex signal beingreceived. The range of the swimmer during the time segment t I is about220 feet and during time segment t t, is about feet.

One explanation for the increase in signal level is that the swimmersubstantially increases the effective target strength of the overallensonified area. Consequently, detecting a substantial increase in thereceived signal level over a five or 10 second period definitelyindicates at least in the case of still water that an intrusion into theguarded volume G is occurring, there being a reasonably low probabilityof false alarm. However, in the environment of a stream having a currentand associated turbulence, the ambient received signal includes asubstantial proportion of Doppler-shifted spectra most probablyattributable to reflections from the moving water and discontinuitiestherein, from objects suspended or floating therein and from surfacewaves which substantially bury the increases in signal intensityattributable to an intruding swimmer particularly at ranges beyond 300or 400 feet.

FIGS. 4 and 5 represent mechanical recordings by a wave analyzer of therelative amplitudes of detected Doppler shift spectra in reflections ofa 30 KHz carrier signal received under conditions of a one-knot primarystream current. Ampitude increases in Doopler shift spectra representedby curves S and F are superimposed upon the ambient spectra for a firstcase wherein an underwater swimmer is swimming downstream at a speed ofabout one knot relative to the one-knot current and for a second casewherein an underwater swimmer is floating or drifting with the one-knotcurrent. Since a target moving at a speed of about one knot willintroduce about a 20 HZ Doppler shift into the reflection of a 30 KHzcarrier signal, Doppler shift spectra of the greatest relative amplitudein the detected spectra of the ambient received signal are observable asoccurring at about 20 Hz which corresponds to the velocity of theprimary current. Observable Doppler shift spectra of frequencies lessthan 20 Hz are thought to be attributable to the vertical velocityprofile of the stream wherein water adjacent the bottom is moving muchmore slowly than water adjacent the surface thereof. Because the swimmerin the example of FIG. 4 is proceeding at a speed greater than that ofthe water, Doppler shift spectra of substantially increased. amplitudeon the order of 20 to 30 db are observable in curve S as peaking at afrequency of about 37 40 Hz indicating a Doppler velocity of about twoknots. The increased amplitudes of these spectra are thought to beprimarily attributable to reflections from the chest cavity and head ofthe swimmer. The increased amplitudes on the order of to db of spectraof higher frequencies in the region of 50 to 73 Hz are thought to beattributable to the movement of the arms and legs of the swimmer.

The curve F of FIG. 5 for the floating or drifting swimmer caseillustrates that the maximum spectra amplitude and the most markedincreases on the order of 20 to 30 db in spectra amplitudes areobservable at frequencies of 22 to 26 Hz which is only slightly greaterthan the 20 Hz spectra attributable to the primary stream current.

The slight difference in the frequencies of the peaked spectra isthought to be attributable to slight, virtually unintentional movementof the arms and legs of the drifting swimmer which are necessarilyundertaken in order to: maintain a relatively constant orientation inthe stream 13. Such movements additionally offer one explanation for thedistribution of Doppler shift spectra of increased amplitude in theregion of 40 to 50 Hz.

Referring now to FIG. 6, the detection system further includes a30 KHZoscillator 30 for providing for projection the single-tone carriersignal. The signal is coupled through a transformer 31 having a centertapped secondary and is applied through a conventional impedancematching tuning coil 32 across the transducer of the projector 11.

Some care must be given toward the selection of the frequency'of thecontinuous wave, single-tone carrier signal to be utilized in thedetection system. For example, it is preferred that the continuous wavesignal have a single-tone frequency which lies in a range above thataudible by human beings. However, if the frequency of the signal is toogreat such as above 150 KHz, the range of the system becomes markedlyattenuated for systems of relatively low power thereby impairing itsutility because, for example, of the reduced warning time forstimulating the undertaking of appropriate countermeasures.Additionally, if the signal frequency is too large such as in the rangeof 150 KHZ commonly used in fish detecting systems, smaller nearbyobjects on the order of the size of small fish are detected by thesystem and contribute a substantial amount of unwanted Doppler noise.One satisfactory frequency for the system carrier signal which has beenfound useful in attaining ranges of about 800 feet in still water and300 to 400 feet in moving water is 30 KHz.

It has been ascertained that underwater swimmers find it difficult toperform underwater work such as attaching explosives when the current ismuch greater than one knot. Further, sustained speeds by underwaterswimmers excessive of one knot relative to the water are unusual,particularly when long distances must be traversed. In view of therelatively small Doppler velocities contemplated, the costs of filtersincluded in the signal processor hereinafter described are also factorsto be considered in selecting the appropriate carrier frequency.

The receiver 12 continuously receives the complex interference signalreflected from the guarded volume G. The output signal of the transduceris applied through a second impedance matching tuning coil 33 andthrough a coupling transformer 34 to a bandpass filter 35. The bandpassfilter is of the type having a center frequency of 30 KHz substantiallyequal to that of the carrier signal and the bandwidth of about 200 Hz tothat the Doppler-shifted signal spectra reflected from moving targetsmay be processed.

In order to provide a reference signal, an attenuated sample of thecarrier signal appearing at the wiper arm 36 of a potentiometer 37 isapplied through a resistor 38 for addition to the received signal beingapplied across the primary of the transformer 34. It has been discoveredthat this configuration eliminates second harmonics of the detectedambient Doppler shift spectra thought to be caused by overmodulation ofthe directly reflected carrier signal with Doppler-shifted signals.

The output signal provided by the filter 35 is amplified by a broadbandpass amplifier 41 and is applied to an envelope detector 42 which,in effect removes the carrier signal and supplies a detector outputsignal including the Doppler shift spectra introduced by reflectivetargets moving within the guarded volume G. The detector ouput signal isapplied to a broad bandpass, variable gain controlled amplifier 43 whoseoutput signal amplitude may be adjusted to a selected level as byturning a knob 44. The apparatus also includes an automatic gain controlloop 45 for adjusting the gain of the amplifier 43 to compensate forshifts in the average ambient level of the output signal of the detector42 over long periods of time such as 10 to l5 minutes, It is preferredthat the amplifiers 41 and 43 have substantially flat responses in theregion of 5 to Hz particularly for installations where the streamcurrent is as low as one-fourth knot.

- The gain controlled output signal of the amplifier 43 is appliedthrough a resistor 50 and across a charging capacitor 51 to a meter 52arranged in a display generally represented at 53 which indicates theoverall signal level of the amplified detector output signal includingin composite Doppler shift spectra of interest. It is preferred that thecapacitor 51 have such a value that the signal is, in'effect, integratedover a period such as l to 5 seconds so that spurious signal levelincreases are not displayed. The output signal of the amplifier 43 isalso applied through an adjustable narrow bandpass filter 54 to avariable Doppler velocity display control 55. One satisfactory type offilter is a tenth octave filter which is continuously adjustable as by aknob 56 from a frequency of 5 Hz to 50 Hz and has octave bandwidthswhich vary between about 2 to 5 Hz. The knob position is preferablycalibrated to Doppler velocity or bandpass center frequency by a scalenot shown. The variable Doppler velocity display control includes avariable attenuator 57 which is adjustable by a knob 58 and throughwhich the filtered signal is applied to a charging capacitor 59 whosevalue is selected to enable a display circuit time constant of a valueon the order of 1 second to avoid alarm actuation by spurious signals.The signal appearing across the capacitor 59 is applied both to a meter60 and to an adjustable threshold responsive driver 61 whose relativeresponse level is varied by adjusting a knob 62 and which provides anoutput singal for activating visual alarm 63 in the display 53. Thealarm activating signal from the driver 61 is also applied through an ORgate 64 to an audible alarm device 65 in the display 53. A satisfactorytype of meter 60 is one which may be damped so that, in effect, itintegrates the amplitudes of the frequency spectra passed by the filter54 over a time period such as about seconds. It can be seen, therefore,that by adjusting the knob 58 the attenuator 57 is controllable so thatthe display of the meter 60 can be adjusted to a zero reading. Asatisfactory type of threshold responsive driver 61 is a meter 60 whichhas an indicator carried contact and includes a movable contactpositioned by the knob 62 at a selected reading greater than zero whichfunctions, when the signal amplitude applied thereto increases asufficient degree so that an electrical connection is establishedbetween the indicator arm contact and the movable contact to apply asignal to activate both the visual alarm 63 and the audible alarm 65.

The output signal of the amplifier 43 is also applied to a bank 70 offixed bandpass filters 71, 72, 73, 74 and 75 having respective bandpasscharacteristics which are down 3 db at the lower and upper ends of thefollowing desired respective frequency ranges: lO to Hz, 15 to Hz, 20 to27 Hz, 27 to 36 Hz, and 36 to 48 Hz. The filter bandpass frequencies areselected so that for the 30 KHz frequency of the carrier signalprojected by the projector 11 they correspond to the magnitude of theDoppler frequency shifts attributable to reflective targets movingrelative to the transducer array 10 with the following speed ranges:one-half to three-fourths knots, three-fourths to 1 knot, l to 1 1knots, l /3 to l-4/5 knots, and 1-4/5 to 2-2/5 knots.

The output signals of the filters 71-75 are applied to respectivedisplay control units 76-80, inclusive, which may be of the same type asthe control 55 each including a knob 81 for adjusting the signal levelof the Doppler shift spectra applied to a respective one of the meters82-86 in the display 53. The display controls 76-80 also includeadjusting means represented by knobs 87 each for adjusting the level ofresponse of a threshold responsive driver such as 61 which provides anactivating signal to a respective one of the alarms 91-95 and also tothe OR gate 64 for driving the audible alarm 65.

In general, operation of the detection system of FIG. 6 is relativeuncomplicated so that unskilled personnel may utilize the device. Oncethe projector 11 and receiver 12 have been installed in positionflanking the vulnerable structure to be protected such as the piling 19of the bridge 14 and their beam axes arranged to intersect at theselected point D upstream from the piling 19, the oscillator 30 isactuated to cause the projector 11 to continuously ensonify the guardedvolume G with the 30 KHz carrier signal. The knob 44 of the amplifier 43is adjusted in order to obtain a reading on the meter 52 near the lowend of its scale. Thereafter the knobs 81 of the display control 76-80are individually operated so that each of the meters 82-86 indicates aselected low or minimal reading. It may be necessary to increase theoverall detector output signal level and thereby, the reading on themeter 52 by adjusting the knob 44 so that at least minimal readings areattained on all of the meters 82-86.

It may be desirable to adjust the position of the potentiometer wiprearm 36 to eliminate second order harmonics. This can be done byascertaining the frequencies of the spectra of maximum amplitude as byadjusting the filter 54 while observing the meter and by adjusting theposition of the potentiometer wiper arm 36 while observing theappropriate one of the meters 82-86 displaying the spectra offrequencies which happen to be the second harmonic of that introduced bythe stream current.

Further adjustment of the degree of attenuation by the controls 76-80may be necessary to attain at least minimum readings by the meters82-86. Thereafter, the threshold response levels of the controls 76-80are first adjusted by operating each of the knobs 87 so that the alarms91-95 are not operating during the prevailing ambient conditions and arethen each adjusted so that the respective signal level increasescorresponding to a selected increase such as 15-25 db actuate the alarms91-95 and 65.

The frequencies of the band of Doppler shift spectra passed by thefilter 54 are gradually increased as by turning the knob 56 so that themeter 60 displays a maximum reading. Thereafter, the frequencies arefurther increased until the meter displays a minimum reading. Then thefrequencies are descreased so that the meter 60 displays an amplitudereading which is about half-way between the maximum and minimumreadings. FOr example, referring to FIG. 5, the bandpass characteristicof the filter 54 after the abovedescribed procedure will have beencentered at about 23 to 24 Hz, the maximum reading having been observedfor Doppler shift spectra of about 20 Hz, i.e., that attributable to theprimary stream current. The attenuator 57 is adjusted as by the knob 58so that the meter 60 displays a minimum reading, and the thresholdresponsive driver 61 is adjusted as by the knob 62 so that the alarms 63and 65 will be activated when the Doppler shift spectra passed by thefilter 54 have undergone the selected increase in amplitude.

Referring to FIG. 4, when an underwater swimmer is approaching at aspeed greater than the primary stream current, Doppler shift spectrawill be introduced into the reflected signals which fter filtering willbe passed by the one or more of the filters 71-75 in the bank and causeone or more of the alarms 91-95 tp be activated along with the audiblealarm 65. Of course, the alarms 91, 92 and 93 may not be activated inthe case of a primary stream current of magnitude as great as l knotsince the primary Doppler shift spectra having amplitude increasesattributable to the swimmer are in the 27 to 48 Hz range which arepassed by the filters 74 and to cause actuation of the alarms 94 and 95.

On the other hand, referring to FIG. 5, floating of drifting swimmersare more difficult to detect in that the amplitude increasesattributable to the swimmer are observed primarily in those Dopplershift spectra introduced by the primary current. Hence the filter 54 isadjusted as described above to pass Doppler shift spectra havingfrequencies in the range of about 22 to 24 Hz when there is a primarycurrent of 1 knot corresponding to 20 Hz. When the selected band ofshift spectra have a marked increase in signal level, the alarms 63 and65 will be actuated which will indicate the intrusion of a floating ordrifting swimmer. Of course, the composite spectra reading displayed onthe meter 52 will slightly increase as the underwater target approaches,increasing the effective target strength. Reliance only upon the overallincrease of the reading display by the meter 52 is to be avoided sincethe meter 52 will also register ambient increases in the overall signallevel, as when the surface becomes rougher or the current increases inintensity.

When the alarm 65 is actuated, the sentry who is operating the systemwill be alerted to refer to the readings of the meters 52, 60, and 82-86and to note which of the alarms 63, and 91-95 are being actuated to ascertain whether the alarm signal is most probably attributable to movingor floating swimmers or to a spurious false alarm. Thereby, theinvention provides a detection system of relatively low cost which maybe operated by unskilled personnel to function as an aid warning ofpossible intrusions of underwater attackers.

The apparatus does not require a complex operating- .procedure' toascertain the received signal content of precise signal levelintensities above a precise reference level of each of a number of exactfrequencies but rather utilizes the relative variations above ambientlevels of a band of spectra which are varied from dayto day or from hourto hour by changing conditions-of current, weather, and water surface.

Obviously many modifications and variations of the present invention arepossible in view of the above teaching. i

What is claimed is:

l. A method of detecting the appoach of an energy reflective objectmoving through a body of water having a current of variable velocitycomprising the steps continually ensonifying a guarded volume of waterwithin the body of water with a single tone acoustic signal to produce areflected interference signal; receiving the interference signalreflected from the guarded volume; processing the received interferencesignal both to extract a plurality of fixed groups of Doppler shiftfrequency spectra of successive frequencies in a plurality of contiguousbandwidths of fixed center frequencies defining in composite apredetermined range of frequency spectra corresponding to a range ofDoppler velocities encompassing the variable velocity of the current andto extract a variable group of Doppler shift frequency spectra in anarrow bandwidth of selectively adjustable center frequency within thepredetermined range; selecting for extraction a variable group ofDoppler shift spectra having frequencies greater than an amplitudepeaked variable group of spectra having a maximum composite signal leveland having a composite group signal level under prevailing ambicntconditions which is less than that of the amplitude peaked variablegroup; and v detecting group signal level increases above presetthreshold levels for each of the fixed and selected variable groups ofspectra.

2. A method according to claim 1 wherein the step of selecting includesthe steps of:

detecting a maximum composite group signal level of a first variablegroup of spectra encompassing those spectra of maximum amplitude whichcorrespond to the prevailing ambient velocity of the current;

detecting a minimum composite group signal level of a second variablegroup of spectra having greater frequencies than the the first group;and

selecting for extraction a third variable group of spectra having bothfrequencies and a composite group signal level intermediate those of thefirst and second variable groups.

3. A method according to claim 2 further comprising the step of:

establishing the guarded volume of water remotely of a location ofreceiving the interference signal.

4. A method according to claim 2 further comprising the step of:

establishing the preset threshold level for each of the fixed and thethird variable groups of spectra above the composite group signal levelthereof which prevails under ambient conditions.

5. A method according to claim 1 further comprising the steps of:

positioning within the body of water and in fixed relation to a bottomboundary of the water both a narrow beam directional projector forprojecting the acoustic signal and a narrow beam directional receiverfor receiving the interference signal at spaced locations along a lineextending transversely of the prevailing direction of the current; and

orienting the projector and receiver to have respec tive beam patternaxes intersecting remotely upstream along the bisector of theprojector-receiver spacing which bisector is aligned with the directionof the current, forming thereby the guarded volume.

6. A method according to claim 5 wherein the steps of positioningcomprises the step of:

spacing the projector from the receiver a distance of at least 10 feetalong the line extending perpendicularly of the direction of thecurrent.

7. A method according to claim 6 wherein said step of continuallyensonifying includes the step of:

continually ensonifying the guarded volume with a single tone acousticsignal having a frequency in the range between audible and KHz.

8. Detection apparatus comprising:

arraymeans including projector means for ensonifying a guarded volume ofwater with an acoustic signal and receiver means for receivingreflections of said acoustic signal and providing a receiver outputsignal;

detector means connected to said receiver means for receiving saidreceiver output signal and providing a detector output signal indicativeof Doppler shift spectra in said receiver output signal;

a plurality of fixed bandpass filters connected to re ceive saiddetector output signal for providing filtered output signals ofrespective frequency bands collectively forming a predeterminedfrequency range encompassing Doppler shift spectra attributable tomoving acoustic signal reflective media;

an adjustable, narrow bandpass filter connected to receive said detectoroutput signal for providing a filtered output signal and havingadjusting means for shifting its bandpass characteristics and centerfrequency over a predetermined range; and

indicator means for indicating signal level increases in the filteredoutput signals of each of said adjustable bandpass filter and said fixedbandpass filters.

9. Detection apparatus according to claim 8 wherein:

said projector means includes a projector and oscillator means connectedto said projector for providing thereto a continuous wave, single tone,signal for projection to ensonify the guarded volume.

10. Apparatus according to claim 8 further comprising:

circuit means connected between said oscillator means and said receivermeans for continuously applying an electrical sample of said single tonesignal for addition to the said electrical output signal of saidreceiver means.

11. Apparatus according to claim 8 wherein:

said projector means and said receiver means comprise a narrow beamdirectional projector and a matched narrow beam directional receiverarranged in spaced relation to form a guarded volume spaced from saidprojector and said receiver.

12. Apparatus according to claim 11 wherein:

said transducer and said receiver are spaced apart along a lineextending transversely of a prevailing current in a guarded volume andhave their beam axes orineted to intersect at an upstream point on acurrent-flow direction aligned bisector of the spacing between saidprojector and receiver.

13. Apparatus according to claim 12 wherein:

said projector means includes oscillator means providing a continuouswave, single tone signal for projection having a frequency in the rangebetween audible and 150 KHz.

14. Apparatus according to claim 12 wherein:

said projector and said receiver each comprise a concave parabolic'signal reflector and a spherical transducer fixed at the focus thereof.

15. Apparatus according to claim 12 wherein:

said projector means includes an oscillator providing a continuous wavesingle tone electrical signal, a coupling transformer having a primarywinding connected to receive said electrical signal and having agrounded, center tapped secondary winding connected to said projectorand a potentiometer having a movable wiper arm connected across saidsecondary winding of said coupling transformer; and

said apparatus further comprises circuit means for applying theelectrical signal appearing at said wiper arm to said receiver means foraddition to attenuator having adjusting means and being con-- 15 nectedto receive said filtered output signal from a respective one of saidfixed and adjustable filters for providing a respective filteredattenuated signal each said control means further including thresholdresponsive alarm driving means connected to receive said filteredattenuated signal for providing an alarm signal;

a plurality of meter means each connected to receive a respective saidfiltered attenuated signal; and

a plurality of indicator means each connected to said control means forreceiving a respective alarm signal and being responsive thereto forindicating that said filtered attenuated signal exceeds a thresholdlevel.

18. Apparatus according to claim 17 wherein said plurality of saidindicator means further comprises:

a plurality of visual alarms each connected to a respective alarmdriving means to receive said alarm signal for providing a visualindication;

OR gate means connected to each of said alarm driving means forreceiving said alarm signals and providing an OR gate output signal; and

audible alarm means connected to receive said OR gate output signal foremitting an audible alarm.

19. Apparatus according to claim 18 further comrpisa resistor connectedat one end to receive the output signal of said detector means;

a charging capacitor connected to' the other end of said resistor forintegrating said signal over a period of at least 1 second; and

a meter connected to said charging capacitor for indicating the level ofthe signal appearing across said capacitor. 0

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,740,704 Dated September 12, 1973 Inventor(s) Henry Suter 7 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 11, Claim 10, line 9, "Apparatus according to claim 8 furthercompris-" should read --Apparatus according to claim 9 compris- Column11, Claim 12, line 26 "axes orineted to intersect at an upstream pointon" should read --axes oriented to intersect at an upstream point 0Signed and sealed this 18th day of December 1973.

(SEAL)- Attest:

EDWARD M, FLETCHER, JR. RENE D. TEGTMEYER Atte sting Officer ActingCommissioner of Patents :fnreM urn-1050 (10-69) USCOMM-DC scans-PasUNITED STATES PATENT OFFICE v CETIFECATE OF CORRECTION Patent No-3,740,704 t d September 12, 1973 Inventor(s) Henry Suter It is certifiedthat error appears in the aboveidentified patent and that said LettersPatent are hereby corrected as shown below:

Column 11, Claim 10, line 9, "Apparatus according to claim 8 furthercompris-" should read --Apparatus according to claim 9 compris- Column11, Claim 12, line 26, Taxes orineted to intersect at an upstream pointon" should read --axes oriented to intersect at an upstream point on--.

Signed and sealed this 18th day of December 1973.

(SEAL)- Attest:

EDWARD M. FLETCHER, JR. RENE D. TEGTI IEYEE Attesting Officer ActingCommissioner of Patents c Dru-1050 (10-69) USCOMWDC 603764,

1. A method of detecting the appoach of an energy reflective objectmoving through a body of water having a current of variable velocitycomprising the steps of: continually ensonifying a guarded volume ofwater within the body of water with a single tone acoustic signal toproduce a reflected interference signal; receiving the interferencesignal reflected from the guarded volume; processing the receivedinterference signal both to extract a plurality of fixed groups ofDoppler shift frequency spectra of successive frequencies in a pluralityof contiguous bandwidths of fixed center frequencies defining incomposite a predetermined range of frequency spectra corresponding to arange of Doppler velocities encompassing the variable velocity of thecurrent and to extract a variable group of Doppler shift frequencyspectra in a narrow bandwidth of selectively adjustable center frequencywithin the predetermined range; selecting for extraction a variablegroup of Doppler shift spectra having frequencies greater than anamplitude peaked variable group of spectra having a maximum compositesignal level and having a composite group signal level under prevailingambient conditions which is less than that of the amplitude peakedvariable group; and detecting group signal level increases above presetthreshold levels for each of the fixed and selected variable groups ofspectra.
 2. A method according to claim 1 wherein the step of selectingincludes the steps of: detecting a maximum composite group signal levelof a first variable group of spectra encompassing those spectra ofmaximum amplitude which correspond to the prevailing ambient velocity ofthe current; detecting a minimum composite group signal level of asecond variable group of spectra having greater frequencies than the thefirst group; and selecting for extraction a third variable group ofspectra having both frequencies and a composite group signal levelintermediate those of the first and second variable groups.
 3. A methodaccording to claim 2 further comprising the step of: establishing theguarded volume of water remotely of a location of receiving theinterference signal.
 4. A method according to claim 2 further comprisingthe step of: establishing the preset threshold level for each of thefixed and the third variable groups of spectra above the composite groupsignal level thereof which prevails under ambient conditions.
 5. Amethod according to claim 1 further comprising the steps of: positioningwithin the body of water and in fixed relation to a bottom boundary ofthe water both a narrow beam directional projector for projecting theacoustic signal and a narrow beam directional receiver for receiving theinterference signal at spaced locations along a line extendingtransversely of the prevailing direction of the current; and orientingthe projector and receiver to have respective beam pattern axesintersecting remotely upstream along the bisector of theprojector-receiver spacing which bisector is aligned with the directionof the current, forming thereby the guarded volume.
 6. A methodaccording to claim 5 wherein the steps of positioning comprises the stepof: spacing the projector from the receiver a distance of at least 10feet along the line extending perpendicularly of the direction of thecurrent.
 7. A method according to claim 6 wherein said step ofcontinually ensonifying includes the step of: continually ensonifyingthe guarded volume with a single tone acoustic signal having a frequencyin the range between audible and 150 KHz.
 8. Detection apparatuscomprising: array means including projector means for ensonifying aguarded volume of water with an acoustic signal and receiver means forreceiving reflections of said acoustic signal and providing a receiveroutput signal; detector means connected to said receiver means forreceiving said receiver output signal and providing a detector outputsignal indicative of Doppler shift spectra in said receiver outputsignal; a plurality of fixed bandpass filters connected to receive saiddetector output signal for providing filtered output signals ofrespective frequency bands collectively forming a predeteRminedfrequency range encompassing Doppler shift spectra attributable tomoving acoustic signal reflective media; an adjustable, narrow bandpassfilter connected to receive said detector output signal for providing afiltered output signal and having adjusting means for shifting itsbandpass characteristics and center frequency over a predeterminedrange; and indicator means for indicating signal level increases in thefiltered output signals of each of said adjustable bandpass filter andsaid fixed bandpass filters.
 9. Detection apparatus according to claim 8wherein: said projector means includes a projector and oscillator meansconnected to said projector for providing thereto a continuous wave,single tone, signal for projection to ensonify the guarded volume. 10.Apparatus according to claim 8 further comprising: circuit meansconnected between said oscillator means and said receiver means forcontinuously applying an electrical sample of said single tone signalfor addition to the said electrical output signal of said receivermeans.
 11. Apparatus according to claim 8 wherein: said projector meansand said receiver means comprise a narrow beam directional projector anda matched narrow beam directional receiver arranged in spaced relationto form a guarded volume spaced from said projector and said receiver.12. Apparatus according to claim 11 wherein: said transducer and saidreceiver are spaced apart along a line extending transversely of aprevailing current in a guarded volume and have their beam axes orinetedto intersect at an upstream point on a current-flow direction alignedbisector of the spacing between said projector and receiver. 13.Apparatus according to claim 12 wherein: said projector means includesoscillator means providing a continuous wave, single tone signal forprojection having a frequency in the range between audible and 150 KHz.14. Apparatus according to claim 12 wherein: said projector and saidreceiver each comprise a concave parabolic signal reflector and aspherical transducer fixed at the focus thereof.
 15. Apparatus accordingto claim 12 wherein: said projector means includes an oscillatorproviding a continuous wave single tone electrical signal, a couplingtransformer having a primary winding connected to receive saidelectrical signal and having a grounded, center tapped secondary windingconnected to said projector and a potentiometer having a movable wiperarm connected across said secondary winding of said couplingtransformer; and said apparatus further comprises circuit means forapplying the electrical signal appearing at said wiper arm to saidreceiver means for addition to said receiver output signal. 16.Apparatus according to claim 12 further comprising: variable gainamplifier means for amplifying said detector output signal applied tosaid filters including an automatic gain control loop for adjusting thegain of said amplifier to compensate for changes in the prevailingsignal level of the detector output signal gradually occurring over aperiod of at least ten minutes.
 17. Apparatus according to claim 12wherein said display means comprises: a plurality of control means eachincluding an input attenuator having adjusting means and being connectedto receive said filtered output signal from a respective one of saidfixed and adjustable filters for providing a respective filteredattenuated signal each said control means further including thresholdresponsive alarm driving means connected to receive said filteredattenuated signal for providing an alarm signal; a plurality of metermeans each connected to receive a respective said filtered attenuatedsignal; and a plurality of indicator means each connected to saidcontrol means for receiving a respective alarm signal and beingresponsive thereto for indicating that said filtered attenuated signalexceeds a threshold level.
 18. Apparatus according to claim 17 whereinsaid plurality of said indicator means further comprises: a plurality ofvisual alarms each connected to a respective alarm driving means toreceive said alarm signal for providing a visual indication; OR gatemeans connected to each of said alarm driving means for receiving saidalarm signals and providing an OR gate output signal; and audible alarmmeans connected to receive said OR gate output signal for emitting anaudible alarm.
 19. Apparatus according to claim 18 further comrpising: aresistor connected at one end to receive the output signal of saiddetector means; a charging capacitor connected to the other end of saidresistor for integrating said signal over a period of at least 1 second;and a meter connected to said charging capacitor for indicating thelevel of the signal appearing across said capacitor.